But some common makeup habits (like borrowing concealer or not cleaning your products regularly) can open you up to a host of skin and health problems.

We reached out to Dr. Kelly Reynolds, germ expert and program director of environmental health sciences at the University of Arizona, and Dr. Whitney Bowe, board-certified dermatologist in New York City, to find out more about germs and your makeup.

1.Makeup harbors bacteria, viruses, and fungi that can lead to serious infections and illness.

"These different kinds of germs often live all over your makeup and brushes," Reynolds tells BuzzFeed Life. She says that common germs include Pseudomonas Aeruginosa, Staphylococcus, and even Methicillin-Resistant Staphylococcus Aureus (MRSA). Because makeup is often waxy and moist, it's the perfect environment for germs to hang out, Reynolds says. Not only can these germs cause serious infections and illness, but they can also worsen acne and other skin problems.

2.Every time you touch your makeup, you're transferring germs that will then go into your mouth, eyes, and any open skin.

Caroline Kee / Via BuzzFeed

"Our hands are covered in these organisms," says Reynolds. It is safe to assume, she says, that the average person is not diligently washing and sanitizing their hands before and between using products. Because you touch your makeup so often and everyone harbors bacteria on their skin, Bowe says it is extremely difficult to tell which makeup products are contaminated and which are safe. But it's not just that you're transmitting germs to your makeup — you're then getting that bacteria-ridden makeup in your mouth, eyes, and any broken skin on your face.

3.Even when you aren't using your makeup, it can pick up fungal spores floating around in the air.

Obviously you're exposing your makeup to germs when you let your foundation brush sit on a dirty bathroom counter. However, Reynolds says that most people forget about all of the contaminants in the air such as dust, and even particles from the cleaning products you use on your mirror. "Fungal spores are everywhere," says Reynolds, and they're especially bad in the bathroom. All they need is a moist place to live, so they'll find their way into that new vamp lipstick or creamy eyeshadow you forgot to close properly.

The good thing is that most cosmetics come with preservatives that help kill bacteria and spores in water-based makeup products so they're less likely to cause an infection, says Bowe. However, these can only do so much, so it's still important to clean and replace your makeup regularly — but we'll get to all of that in a bit.

4.All the germs and dirt on your brushes and sponges can wreak havoc on your face.

"Frequently used makeup brushes act as breeding grounds for germs," says Bowe. "They're run over an oily face many times, which spreads bacteria around." Old brushes may also contain dried-up product on the bristles, which can cause the brush to be abrasive and cause skin irritation or redness. Even if you're applying makeup to your freshly washed face, those brushes can still deliver germs onto your skin.

The best way to combat this is to clean your brushes properly every week with at least alcohol or baby soap, says Reynolds. We'll go into more detail about best cleaning practices in a little bit. Another option is to use sterile disposable cotton balls or pads instead of brushes to reduce contamination. It may be wasteful, but it would be worth it, especially if you have any sores on your face or had an infection recently, says Reynolds. And obviously, never share makeup brushes.

5.An entire jar of powder can become contaminated after just one use.

"Absorbent or loose powders are especially difficult," says Reynolds. "One use with a dirty brush can contaminate the whole jar with germs," Reynolds says. "If you need to share it, have a skin infection, or chronic acne — allocate some of the loose powder into a separate dish or plastic cup, which you can easily clean, and use it from there instead." This helps reduce the risk of germs in the main supply of powder and cross-contamination from multiple people using one product.

6.Sharing makeup means you're literally sharing germs.

Caroline Kee / Via BuzzFeed

If you let your friend borrow your products, you're cross-contaminating them with all of the germs from that friend's hands and face, says Reynolds. And ditto if you dabble in someone else's makeup stash. This is especially concerning if either person has any broken skin or wounds on their face, which can both spread and host infections. "I tell patients to never share anything that comes into direct contact with another person's skin or mucus membranes," says Bowe.

"People don't realize that staph bacteria are super common," Reynolds says. "Someone can carry the bacteria on their body with no adverse affect and spread it to makeup, then the next person who uses it can get a bad staph infection under the right conditions."

7.Using contaminated lipstick is like licking a surface covered in bacteria and viruses — even herpes.

"Bacteria can transfer through the membranes of your mouth into your bloodstream," says Bowe. Lips have an extensive network of blood vessels just under the surface, she says, ready to absorb anything you apply on the mouth, germs included. Plus, viruses can survive for weeks on lipsticks thanks to the waxes and proteins in them.

"The contagious herpes simplex virus, which causes cold sores, can be spread between two people sharing lipstick, even if the infected person has no visible sores," says Bowe.

8.Makeup testers are innocent-looking petri dishes full of bacterial and fungal growths from random people.

Yes, it's totally free, high-end makeup just sitting there waiting for you to use — but is it worth the risk? Probably not. "Studies have found bacteria such as staph, strep, and E. coli can be spread when people don't wash their hands after using the restroom and then touch makeup store surfaces and testers," Bowe says.

Even if the store provides brand-new brushes and applicators to try the makeup, the sheer number of people using a product causes it to become highly contaminated. Reynolds suggests avoiding testing anything from makeup counters at all costs. "So many people are coming in and out and it's impossible to know what they've done before you."

9.Sharing makeup — or even just using your own contaminated makeup — can make acne worse.

When you cover up a pimple, you're actually targeting broken skin, which is the most vulnerable part of your face for bacteria to enter and breed, says Reynolds. So even a minor pimple can become infected from contaminated makeup. Another reason why you shouldn't share makeup is because you can transfer acne-causing bacteria from one face to the next. Acne is not "contagious," says Bowe, but the contaminated makeup changes the environment on your skin by adding bacteria, which can make you more prone to breakouts or worsen existing acne.

If you have acne, Bowe suggests always washing your hands and face super well before applying makeup, using clean brushes, and dabbing concealer on bad spots with a clean Q-tip instead of touching any product to your face directly.

10.Because your eyes are the most vulnerable to infections, sharing eye makeup is especially dangerous.

We're talking pinkeye, blepharitis (inflammation of the eyelid), or the formation of a stye, says Bowe. Eyelash mites can also stick to mascara wands and transfer to the next person who uses it.

It's even worse if you have contacts. "It's much easier for germs to infect your eye because they get trapped beneath the lens," says Reynolds. This makes it more difficult for the eye to wash out contaminants. Plus, sometimes the fungus in your makeup mentioned earlier can get trapped and grow where it wouldn't otherwise, she says.

11.Pinkeye germs can survive in your makeup for a LONG time and even reinfect you after healing.

Conjunctivitis (pinkeye) can actually be transmitted by germs in your eye makeup if you've shared it with someone who has the bacteria or virus. The tricky part is that people can be contagious and have zero symptoms if they're either about to get pinkeye or they're still recovering. "It's just better to play it safe and never share eye makeup," says Reynolds.

If you have pinkeye, Reynolds says it's crucial to stop wearing makeup until it's healed and throw away any makeup used on your infected eye, because you can actually reinfect yourself after you've healed.

12.Germs multiply over time, so old makeup puts you at serious risk.

Jenny Chang / Via BuzzFeed

"Most people keep their makeup long past the expiration date," Reynolds says. "The number of germs increases over time and they continue to go on your skin each time you apply." Ideally, you should replace most makeup every six months, and eye makeup even more frequently. "Replacing your eye makeup every three to six months may seem too frequent or difficult, but consider your health and hygiene first," she says.

Unfortunately, in the United States there's no law requiring cosmetic manufacturers to print expiration dates on products. However, you can find this information from a variety of sources online.

13.So you should probably be cleaning and sanitizing your makeup way more than you already do.

It's easy, cheap, and always worth it. Bowe suggests cleaning makeup brushes and sponges with baby or antibacterial soap on a weekly basis. This will remove bacteria accumulated from touching your skin and other surfaces, and dust or other particles from the air. You should replace your brushes when the bristles start to feel hard and stick together.

You can also use alcohol to disinfect pressed powder, eye makeup, lipstick, and more. "Wiping a layer of lipstick off with a tissue isn't doing enough to remove the germs," says Reynolds. The best way to sanitize lipstick is by gently wiping it off with a cotton ball soaked in rubbing alcohol. If you find yourself super busy, Reynolds suggests keeping a small spray bottle filled with alcohol in your makeup cabinet so you can quickly spritz any makeup product or brush to disinfect before use.

Antibacterial wipes are also perfect for wiping the tip of the pump for creams or liquid foundations, and cleaning the inside of your makeup bag. Find more genius ways to clean your makeup here.